In the drilling or producing of subsea offshore wells, particularly in deep waters, several operating conditions are necessary. First, the drilling or production ship or vessel must be continuously maintained approximately over the drill or well site in spite of prevailing surface conditions. Secondly, the elongated mud riser or production riser which communicates the vessel with the well head must be maintained relatively straight and is therefore constantly uder tension. This riser is maintained in the state of tension by either an active or passive system during the drilling or producing operation. Thus the riser maintains a substantially static condition in spite of movement of the vessel, whether laterally or in a vertical direction.
In the instance of the drilling riser, tension is normally applied to the upper end thereof from suitable active tensioning apparatus carried on the drilling vessel. The usual apparatus for the purpose is responsive to any movement of the vessel. Thus, in effect both the upper and lower ends of the riser are axially tensioned, even though pivotally connected. The remainder of the riser is subjected to both bending and axial stresses.
In the instance of the production riser one or both ends of the unit can be fixed against rotation in order to eliminate maintenance problems on underwater pivotal joints.
In both the drilling and production risers, a telescoping joint is provided at either the top or the bottom of the unit to permit the vessel to move without inducing additional stresses into the riser. This arrangement assures that tension in the riser will be maintained constant regardless of the complex motions to which the surface vessel is ordinarily subjected under severe weather conditions.
In one form of marine drilling vessel, notably the semisubmersible type, the effects of weather conditions on the vessel's motions are minimized by the vessel's wave transparent structure. Thus, in the ordinary semisubmersible unit, the vessel support elements comprise a plurality of upstanding column-like members which support the working deck above the water's surface. Said upstanding members are controllably buoyant to regulate the vessel's vertical orientation in the water.
This type of vessel has been found to operate with particular effectiveness in harsh environments characterized by the North Sea area. In such areas, a prevailing problem is the severe wind, waves and currents which disrupt drilling or producing operations due to stress limitations in the risers. While normal drill ships experience considerable down time as a result of excessive motions of the ship in a harsh environment, in the instance of semisubmersible vessels, the wave transparent structure assures a minimal amount of resistance to waves.
Understandably then, under aggravated storm conditions the motion of the vessel will be greatly reduced. Thus, while the structure remains relatively stable in the water, the waves will tend to pass through the support columns and exert only a limited lateral force thereon. However, even with semisubmersible vessels considerable down time or non-operating time is experienced due to the weather at such time as the risers become overstressed, as for example when waves achieving approximately 30 to 35 feet in height are present.
Semisubmersible vessels of the type contemplated are normally maintained in position by anchoring rods extending outward and downward from the vessel to the ocean floor. Thus, although the vessel will be deflected somewhat from its position in response to the force of wind, waves and current, such deflection will be minimized. Maintaining the mud riser in the desired condition then will be readily achieved for either drilling or producing operations.
Another type of vessel which offers great potential in deep water is the tension leg platform. This structure is similar in many respects to the semisubmersible type vessel above mentioned. The mooring lines on the tension leg unit, however, extend vertically, instead of with the normal catenary used in conventional mooring processes. By ballasting the vessel to the desired degree the mooring lines can be maintained under constant tension. Thus, heave, roll and pitch of the vessel are virtually eliminated.
Although this tension leg type vessel will permit personnel to continue working under more severe sea conditions than either a conventional ship or semisubmersible vessels, it will nonetheless also be subjected to considerable down time due to weather prompted by the stress limitations of the drilling or production risers.
Considering the ordinary profile of an ocean wave, particularly when generated under storm conditions, the maximum lateral force of the wave will be exerted near the wave surface. This force will progressively decrease with the depth of the water. The same type lateral force profile is exhibited by ocean currents during storms and the like.
It has been demonstrated that mooring systems on semisubmersible vessels can be adequately designed to maintain the latter on location even during the most severe storms with the rig being little affected. In contrast, however, the effect of storm waves and currents on the mud riser is so severe that it is found impossible to maintain the risers connected to blowout preventers or the like during extreme stormy conditions. Breaking risers can be both expensive and time consuming, which characteristics in the instance of offshore operations, are considered synonymous.
Toward overcoming this concentration of stress in the mud riser during stormy conditions, a drilling system is provided as herein described, for operating in deep offshore waters. Said system is particularly adapted to permit the vessel to function continuously in spite of the water conditions.
The drilling rig comprises normally a semisubmersible or tension leg type of drilling or production vessel as herein mentioned. Said rig includes pipe drilling or production risers extending downwardly therefrom and function to protect the drill string, as well as to conduct drilling mud or oil and gas between the vessel and a well. The elongated riser extending from the blowout preventer or well head to the vessel, is fastened at opposite ends. It is further tensioned at the upper end to permit drilling and/or producing operations to continue under all circumstances.
To minimize the stress induced into the riser during severe turbulent water conditions, a downwardly extending rigid caisson depends from the drilling or production vessel. Said caisson encloses the upper segment of the riser along that length thereof most susceptible to high stress concentrations. The caisson thus forms a confined though open area about the riser, permitting a liberal degree of riser deflection as the semisubmersible adjusts its position. However, the sytem allows for free movement of the blowout preventer and well head equipment through the caisson at such times as the latter must be raised or lowered.